General Relativistic Magnetohydrodynamic Simulations of Black Hole Accretion Disks: Results and Observational Implications

A selection of results from the general relativistic MHD accretionsimulations described in the previous talk are presented. We find that themagnetic field strength increases sharply with decreasing radius and is alsoenhanced near rapidly-spinning black holes. The greater magnetic field strengthassociated with rapid black hole rotation leads to a large outwardelectromagnetic angular momentum flux that substantially reduces both the meanaccretion rate and the net accreted angular momentum per unit rest-mass. Thiselectromagnetic stress strongly violates the traditional guess that theaccretion stress vanishes at and inside the marginally stable orbit. Possibleobservational consequences include a constraint on the maximum spin of blackholes, enhancement to the radiative efficiency, and concentration offluorescent Fe Kalpha to the innermost part of the accretion disk.Comment: invited review at the conference "Stellar-mass, Intermediate-mass, and Supermassive Black Holes", held in Kyoto, Japan, Octorber 28-31, 2003, to be published in Progress of Theoretical Physics Supplemen